Hydrogen activation on Fe3O4(110): From molecular H2 to atomic H under reducing conditions

Hydrogen reduction of Fe3O4 plays a pivotal role in sustainable steelmaking, offering a low-carbon alternative to traditional carbothermic processes. In this study, we employ density functional theory to investigate the dissociative adsorption of molecular H2 and the subsequent adsorption behavior of atomic H on Fe3O4 (110) surfaces, considering both stoichiometric and O-deficient configurations. Our results reveal that the type and location of O vacancies critically influence both the thermodynamics and kinetics of H2 activation. Compared to the perfect surface, the presence of O vacancies increases the activation barrier for H2 dissociation. Twofold coordinated O sites, which are thermodynamically more favorable to form, reduce the reaction exothermicity. Conversely, threefold coordinated O vacancies, though less readily formed, stabilize the dissociated state more strongly but incur the highest activation barrier. For atomic H adsorption, adsorption is strongly favored at O sites over Fe, particularly at hollow sites adjacent to twofold O ions. While O vacancies themselves are not favorable adsorption sites, they alter the local electronic environment and change the H binding strength. Bonding strength, quantified via the integrated crystal orbital Hamiltonian population, shows a strong linear correlation with H adsorption energies across all surface types. This correlation underscores the critical role of H-O orbital hybridization in stabilizing adsorbed H species and provides a quantitative link between adsorption strength and the underlying surface-adsorbate bonding characteristics. Our results offer atomic-level insights into defect-mediated H2 activation and H adsorption on Fe3O4, with implications for advancing hydrogen-based processes in steel production, hydrogen storage, and heterogeneous catalysis.